1. Field of the Invention
The present invention relates to a safer and more efficient method of synthesizing the primary detonator material copper(I) 5-nitrotetrazolate (DBX-1) starting with 5-aminotetrazole (5-AT); wherein, the intermediate materials of an acid copper salt of 5-nitrotetrazol (5-NT) and sodium 5-nitrotetrazolate (NaNT) are free of the starting material 5-AT—a material which inhibits the formation of the DBX-1.
2. Related Art
In military and commercial blasting, the explosive chain reaction is typically initiated by detonation of a small quantity of a highly sensitive primary explosive material. The sensitive nature and significant explosive effect of such primary explosive materials, allows for significantly larger quantities of relatively insensitive secondary high explosive material to be detonated using a very small quantity of the very sensitive primary explosive material—thereby minimizing the overall sensitivity of the explosive system. Two of the most widely used primary explosives are lead(II) azide (“LA”) and lead(II) styphnate—which, due to their lead content, have significantly contaminated the air and soil about military training grounds and government and commercial firing ranges—posing a hazard to the safety of the personnel working thereabout, as well as, the environment in general. In addition, LA is problematic because the azide anions can react with moisture in the presence of carbon dioxide to generate hydrazoic acid, a toxic and explosive material. Further, azide can also form extremely sensitive explosive complexes with other metals, such as copper. The unintended formation of copper azides in aging munitions with copper detonator shells has let to fatal accidents as bomb investigators and explosive ordinance disposal teams have attempted to move such items.
Considering the above drawbacks of LA and lead(II) styphnate, as detailed above, there has been a focus on developing alternative minimally toxic and chemically inert primary explosives. One promising alternative primary explosive material is copper(I) 5-nitrotetrazolate (aka “DBX-1”)—which has proven to be a drop-in replacement for LA in existing detonator designs. DBX-1 has comparable properties to LA as an explosive; but, does not have the toxicity or other drawbacks that LA suffers from. In spite of this, DBX-1 has made little progress in replacing LA, due to issues with its production.
Typically DBX-1 is prepared from sodium 5-nitrotetrazolate (NaNT), a compound that has been used as a precursor to other explosives which are produced on large scale, including tetraamine-cis-bis(5-nitrotetrazolato)cobalt(III) perchlorate (BNCP), and mercuric nitrotetrazolate. The synthesis of NaNT and subsequently DBX-1 is shown in Scheme 1, below,

This synthesis of the NaNT was first disclosed by von Herz in 1937, wherein 5-aminotetrazole (5-AT) is first converted to 5-nitrotetrazole (5-NT) by means of a Sandmeyer reaction. The product precipitates from the reaction mixture as 1 (or Complex 1 or intermediate Complex 1), i.e. a copper(II) complex sometimes referred to as the ‘acid copper salt’ of 5-NT. Von Herz reported its composition as Cu(H—NT)(NT)2(OH2)4. This material is a gelatinous green solid that is isolated by a tedious filtration. In 1978, Gilligan and Kamlet in U.S. Pat. No. 4,093,623 disclosed that this filtration became much easier using modified conditions for the Sandmeyer reaction. Specifically, dosing 5-AT in dilute nitric acid instead of dilute sulfuric acid as originally reported led to a dramatic reduction in the amount of time required for the isolation and washing of 1. Gilligan and Kamlet also eliminated “microdetonations” that accompanied dosing 5-AT using the von Herz procedure. These microdetonations were postulated to be due to gaseous nitrous acid reacting with the 5-AT to generate the diazonium species before mixing with the solution containing the catalyst. Without a catalyst present, this highly unstable material would decompose rapidly. Gilligan and Kamlet solved this problem by adding a small quantity of a copper(II) salt, CuSO4, to the solution of 5-AT. This catalyzed the reaction of the diazonium species before it could accumulate and detonate and this procedure is widely used today.
Looking more closely at the present typical process, i.e. the prior art method, the nitro group in 5-NT is installed by a Sandmeyer reaction of 5-aminotetrazole, accomplished by dosing a solution of 5-AT to a mixture containing sodium nitrite and a copper(II) catalyst. This converts 5-AT to 5-NT, which precipitates from the reaction mixture as complex 1, a gelatinous solid. When performing this reaction, the dose solutions of certain concentrations of 5-AT in nitric acid resulted in the nitrate salt of 5-AT precipitating in the dosing lines. 5-AT nitrate is an impact-sensitive explosive reported to melt with violent decomposition at ca. 170° C. In addition to the safety concerns in having this energetic material accumulate in the dosing lines, solid 5-AT nitrate being dosed to the copper sulfate/sodium nitrite solution could result in the precipitation of copper(II) complexes of 5-AT. If these complexes precipitate from solution at this point in the reaction, they would be isolated along with the copper(II) 5-NT Complex 1. As Complex 1 is converted to the NaNT, the copper(II) 5-AT complexes would be converted back to free 5-AT and remain in solution as an impurity, making additional purification steps necessary before the NaNT could be used to produce DBX-1—as any free 5-AT will inhibit the production of DBX-1.
Further, in the typical prior art method detailed above, Complex 1 is a sensitive explosive when dry, and manipulating this material presents hazards to operators, especially as the production scale increases. Again, typically, in a second step, 1 is manually removed from the filter and charged to a reactor containing water. The resulting slurry is treated with aqueous sodium hydroxide, leading to the precipitation of copper(II) oxide, which is removed by filtration, leaving a solution of sodium 5-nitrotetrazolate (NaNT). The resulting solutions of NaNT typically contain a significant amount of residual 5-AT as an impurity (ca. 5% by weight of total dissolved species)—as stated above, such free 5-AT inhibits the subsequent production of DBX-1. So, to purify this material for conversion to transition metal 5-NT complexes, NaNT is typically isolated as its dihydrate, which becomes extremely sensitive if it is allowed to convert to the anhydrous form in dry air. This purification adds another operation to the procedure, adding cost and complexity to the process, and further increasing operator exposure to sensitive energetic solids.
The chemistry involved in the conversion of NaNT to DBX-1 is known in the art. The particulars of this process where disclosed in a paper by John W. Fronabarger et al, published in Propellants, Explosives, Pyrotechnics, Vol. 36, pages 541-550, 2011 and in U.S. Pat. No. 8,523,989, granted to Fronabarger et al, on Sep. 3, 2013.
Thus there is a need in the art for a synthesis of NaNT that can be converted to DBX-1 without any 5-AT contamination, such that no unsafe and inefficient purification is required to remove such a 5-AT contaminant that will inhibit the production of DBX-1. Plus, there is a need to avoid removing and handling the key intermediate Complex 1, which in the currently/prior art process requires removal from the reactor to be filtered and washed—a significant safety issue.